1. Field of the Invention
The present invention relates to an optical film by which the optical path of light incident on a side surface of a liquid-crystal display panel is changed to a viewing direction efficiently to thereby make it possible to form a liquid-crystal display device which is thin in thickness and light in weight with display bright, little in disorder of an image and easy to view both in an external light mode and in an illumination mode.
2. Description of the Related Art
As a reflection type liquid-crystal display device which can be used both in an external light mode and in an illumination mode, there has been heretofore known a device in which an optical film having light output portions constituted by fine prisms is stuck onto a visual side surface of a liquid-crystal display panel so that light incident on a side surface of the panel from a light source disposed on the side surface of the panel and transmitted inside the panel is reflected by the light output portions of the optical film to thereby illuminate the liquid-crystal display panel (Unexamined Japanese Patent Publication No. 2000-147499). That is, a system thin in thickness and light in weight for illuminating a liquid-crystal display panel has been achieved by use of an optical film further thinner than a side liquid-guide plate which was used in the related art.
Further, bright display has been achieved by the optical film excellent in directivity of illumination light compared with a related-art reflection type liquid-crystal display device (Unexamined Japanese Patent publication No. Hei. 5-158033) in which light incident on a side surface is totally reflected toward a liquid crystal layer side through a visual side transparent substrate of a liquid-crystal cell and reflected by a roughened surface type reflection plate disposed on a rear surface of the device to thereby form display light. Incidentally, it is difficult to effectively use the reflection from the roughened surface type reflection plate for display because illumination light exits in a direction largely inclined to a frontal direction of the liquid-crystal display panel.
In an LCD (liquid-crystal display device) using the related-art optical film having light output portions constituted by fine prisms, particularly in a front light system for viewing a display image through the optical film disposed on the visual side, there was however a problem that display was dark and contrast was low.